You can use XCopy with the /M switch {xcopy /m d:\source\*.* d:\dest\*.*} which will only copy those files with the Archive Bit set, i.e. those that have changed. It will then reset the Archive Bit. Of course if you have never done this all the files will look like they are changed files.

Update: Then again if you have used a backup program before most of them will clear the Archive Bit.

In my previous post I tried to just answer your question. However, I agree with Paul that RoboCopy is a great program. I have a batch file that runs Robocopy to keep a copy of everything that changes updated on my 2nd internal drive. I have a custom toolbar sitting on the taskbar with shortcut to this batch file 2 clicks and my Documents folder is backuped no fuss no muss. Here's the RoboCopy command I use:
Robocopy "G:\BEKDocs" "V:\BEKDocs" /MIR /XJD /R:5 /W:15 /MT:32 /V /NP /LOG:RoboCopy.log

Note: I've moved my Documents folder off the C: drive so you'd need to adjust accordingly.